Are you considering to jump on the Vista bandwagon in the near future, then remember to apply below HotFix on your Exchange 2003 Server in order to fix any OWA 2003 issues you might run into.
In the upcoming Microsoft Windows Vista Beta 2 release, the Dynamic HTML Editing ActiveX control is being removed from the Internet Explorer browser. As a result, there is functionality missing that Microsoft Exchange Outlook Web Access relies on. This update replaces that deprecated functionality on the Microsoft Exchange servers so that Microsoft Exchange Outlook Web Access continues to function smoothly.
Get it here.